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Taking Control of Screen Coating

(May 2009) posted on Wed May 20, 2009

The stencils you use for printing are only as good as the coating procedures you follow to produce them. Here you

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By KIWO Technical Support Department

The degree of roughness of the stencil surface is measured as an Rz value. Basically, the lower the Rz value, the smoother the surface and, therefore, the better the seal that occurs between the substrate and stencil during printing. By adjusting the coating technique, users of direct emulsions can fine-tune the Rz value of their stencils for any particular substrate. Such adjustments are necessary to achieve accurate and consistent prints on all materials.

The key to proper gasketing between stencil and substrate is for the emulsion to provide a shoulder that maintains space between the substrate and the mesh in open areas of the stencil. Because ink cannot travel through mesh threads, having some emulsion thickness over (on top of) the mesh plane helps to define the edges of the image (Figure 1). (Figure Reference: EOM levels of 10-20% are generally recommended for screen printing (no more than 10% for process-color printing, particularly with UV inks. The extra emulsion thickness provides a shoulder at the mesh openings that keeps the mesh and substrate from making contact, which allows ink to transfer from the entire open area, leading to good image definition and dot shape and size.) Without this emulsion over mesh (EOM), the risk of sawtoothing and dot loss increases, and fine detail printing becomes virtually impossible.


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